This study will attempt to further define the primary factors involved in sodium balance in normal and edematous states. It was originally proposed that the proximal tubule was the main site of sodium retention in edematous states. However, studied in this and other laboratories have recently shown that proximal tubular sodium reabsorption in all likelihood is not significantly enhanced in experimental models of salt retention. Therefore, experiments will be designed to determine the factors which regulate sodium reabsorption in the ascending limb of the Loop of Henle and whether this is the predominant site of sodium retention in edematous states. Both free flow micropuncture and microperfusion techniques will be utilized to evaluate the effect of delivery, volume expansion, alterations in capillary oncotic and hydrostatic pressure, drug-induced vasodilatation, and changes in kidney GFR on sodium and water reabsorption in Henle's loop in normal and salt retaining animals. In addition, alterations in the intrarenal distribution of blood flow has been suggested to be a major determinant of the handling of sodium by the kidney. With the use of the radioactive microsphere technique, studies will be performed to define what factors alter the intrarenal distribution of blood flow and if this mechanism is of importance in the production of sodium retention in edematous states.